The use of mass spectrometer (MS) sample analysis systems, to analyze structure and composition of samples is well known. Generally sample analysis in MS sample analysis systems involves four steps, namely: (1) sample introduction; (2) ion generation; 3) ion separation; and (4) ion detection. By far, the steps most critical to successful MS sample analysis system operation are those concerning sample introduction and ion generation.
In the case where a sample is a gas flow on the order of one (1) mililiter per minute, sample introduction can be a relatively simple matter. Gas phase sample molecules can be directly introduced into the MS ionization chamber and ionized and fragmented therein by for instance: electron impact ionization (EI), chemical ionization (CI), fast atom bombardment (FAB) ionization, or interaction with energy in metastable species or photons, depending on the ionization method used. However, when the sample is a liquid solution, such as conventional liquid chromatography (LC) system effluents, sample introduction becomes much more difficult. Even when conventional LC effluent mililiter-per-minute sample solution flow rates are reduced by a factor of one-thousand (1000) by flow splitting, prior to introduction into a low pressure MS environment, maintaining a ten-to-the-minus-fifth Torr pressure therein has proven to be very difficult when Direct Liquid Injection (DLI) sample introduction systems are used, (see Pullen et al., J. Chromat., 474, p. 335-343, 1989). In addition, flow splitting reduces sample analysis system sensitivity and precludes application to very small sample volumes.
One technique, other than flow splitting and direct liquid injection, directed to solving the problem of introducing liquid sample into low pressure MS ionization chamber or detector elements utilizes a moving belt system. Moving belt systems are discussed by Scott et al., (J. Chrom., 99, p. 394, 1974), and involve transport of sample by a stainless steel wire through a series of vacuum locks in which solvent present is removed by a vaporization process. Residue sample analyte on the stainless steel wire is entered directly into the low pressure MS ionization chamber. Another technique involves use of particle beam generating systems. U.S. Pat. No. 4,968,885 to Willoughby describes a system which utilizes a combined thermal and pneumatic approach to generating nebulized sample particles. Another U.S. Pat. No. 4,977,785 to Willoughby et al. emphasizes aerosol generation by decompression of a liquid or supercritical fluid stream. A patent to Brandt et al. U.S. Pat. No. 4,863,491 describes a system for separating particles from a gas stream in a vacuum chamber through a capillary inlet at supersonic speed. A patent to Browner et al., U.S. Pat. No. 4,629,478 describes a system which forms a stable jet of liquid sample flow at a velocity allowing columnar breakup into droplets of uniform size and spacing. An intersecting gas flow serves to disperse the results. A patent to Dorn, U.S. Pat. No. 4,980,057 describes a system for generating nebulized sample particles from a liquid sample which utilizes both pneumatic and ultrasonic means. U.S. Pat. Nos. 4,883,958 and 4,958,529 to Vestal describe spraying systems which can be used to produce nebulized particles from liquid sample. Separate pressure reducing skimming means are required to prepare and enter particle beams into low pressure MS sample analysis system ionization chamber and detector elements.
Additional techniques for coupling liquid samples into low pressure MS have been derived which greatly increase the analysis capabilities of MS sample analysis systems. The most important of these involve techniques which directly generate ions from liquid samples.
Thermospray systems are an example of such systems, and are described in a paper by Vestal and Blakely, (Anal. Chem., 55, p. 750-754, 1983). This paper describes a thermospray technique in which LC eluent containing a modifier such as ammonium acetate is passed through a heated capillary tube to rapidly evaporate the solvent present. Most of the evaporated solvent is pumped away and ionized sample analytes produced are directed into a MS for detection. A patent, U.S. Pat. No. 4,730,111 to Vestal also describes a thermospray system as do U.S. Pat. Nos. 4,883,958 and 4,958,529 to Vestal.
Other examples of systems which directly generate ions from liquid samples in atmospheric pressure are ionspray and electrospray systems.
A patent to Henion et al. U.S. Pat. No. 4,861,988 describes an ion spray sample nebulizer system which sprays sample into an atmospheric pressure ionization chamber through a stainless steel capillary tube. An impressed electric field produced by application of a plus or minus approximately 3 KV to the stainless steel capillary serves to form ionized particles. Another patent to Henion et al., U.S. Pat. No. 4,935,624 describes a similar system, but provides for application of thermal energy. The later Patent also describes use of the thermally assisted system without application of the plus or minus approximately 3 KV.
A patent to Allen et al., U.S. Pat. No. 5,015,845 describes an electrospray system which produces nebulized ionized sample particles by applying a high voltage to a capillary tube which contains liquid sample, at atmospheric pressure or above. There is no concurrent gas flow, heated or otherwise involved. The electrosprayed droplets produced are passed into a low pressure heated ion generating chamber in which solvent is removed therefrom, to produce ionized molecules for analysis in MS sample analysis systems.
Other techniques for ionizing nebulized sample particles to provide ionized sample molecules and ionized sample fragments etc. prior to entry to a MS low pressure region are identified by the term "Atmospheric Pressure Ionization" (API). API ionization systems include those which utilize Beta Particle Emitters, Corona Discharge and Direct Current Glow Discharge, and Plasmas such as Inductively Coupled Plasmas (ICP), to provide sample molecules and fragments etc. in ionized form.
Papers by Horning et al, (Anal. Chem., 45, p. 936-943, 1973) and (J. Chromatog. Sci., 12, p. 725-729, 1974) describe atmospheric pressure sample ionization systems which utilize both nickle-63, beta-particle emitter and needle to plane corona discharge ion sources.
A paper by Sofer, (Appl. Spec. 44, p. 1391-1398, 1990) describes an atmospheric pressure direct current glow discharge source used as a means to enhance the ionization efficiency and sensitivity of API/MS systems. A glow discharge in one atmosphere helium is described which penetrates into an extended region of space at currents hundreds of times higher than in corona discharge.
A patent to Tsuchiya et al., U.S. Pat. No. 4,546,253 describes a means for producing metastable species at atmospheric pressure by use of a high voltage induced corona discharge from a needle shaped emitter, which during use is situated in a stream of carrier matrix gas. Metastable species produced are directed by the flow of carrier matrix gas to a specimen which becomes ionized as a result of interaction with said metastable species and the presence of the high voltage at the needle shaped emitter. Use with liquid chromatography derived sample solutions and mass spectrometers is also described.
A patent to Dodge III et al., U.S. Pat. No. 4,309,187 describes a system in which nitrgen molecules are excited in a dielectric field to form metastable energy level nitrogen. Said metastable nitrogen is mixed with a gas stream containing atomic species and interaction causes fluorescence thereof, which fluorescence is analyzed to determine the identity and concentration of said sample atoms species. The Dodge III et al. Patent teaches that the nitrogen partial pressure should be at between one (1) and three-hundred (300) Torr during use, hence is not a true API system.
The API Corona Discharge and Direct Current Glow Discharge systems described operate at low power, (e.g. less than 1 watt), or utilize needle shaped electrodes to develop or sustain electric discharge at atmospheric pressure and high voltage. Too high a voltage, however, can cause discharge instability and metastable species created by discharge from a needle point electrode lose energy quickly through ionizing collisions with matrix gas and solvent present in the vicinity of the needle electrode. Sample particles are ionized by interaction with matrix gas and solvent ions, resulting in a soft, chemical, ionization in which very little sample fragmentation occurs.
Continuing, it is advantageous to operate both sample nebulizer and nebulized sample particle ionizing systems at atmospheric pressure because operational, contamination and maintenance problems are reduced. For instance, interfacing to liquid form samples is more convenient at atmospheric pressure. Also, a sample ionization system can be modularly removed from a MS sample analysis system and serviced without requiring its sample analysis detector be brought to atmospheric pressure. As well, where non-ionizing sample nebulizer means are used and produced nebulized sample particles must be ionized, the presence of a higher ambient pressure in the sample ionizing system, (hence greater number of energy containing intermediaries), leads to more ionizing interactions between nebulized sample particles and energy containing ionizing intermediaries.
As alluded to above, during sample analysis it is often desired to determine not only the molecular weight of a sample molecule, but also that of the structural fragments etc. thereof. This is often accomplished presently by use of tandem MS/MS systems in which the first MS stage separates, and isolates a sample molecule of interest and determines the molecular weight thereof. Some of said separated and isolated sample molecules are then subjected to fragmenting forces and the resulting ionized sample fragments etc. produced are entered to the second stage of the tandem MS system in which the ionized fragments are separated and detected. Tandem MS/MS systems, however, can be very expensive. In addition, problems develop during use because of contamination in the low pressure chambers, difficulty in maintaining low pressures in sample ionizing and fragmenting systems, and in maintaining proper alignment of the stages. A system which provides the benefits of tandem MS/MS systems and which operates at atmospheric pressure would therefore be desirable.
As mentioned above, the use of ICP's is an API ionization approach. ICP's have been very successfully used as an ionization source for elemental analysis since a higher power, (e.g. 1000 watts), typically fragments sample particles into elemental ions. However, the parent molecules from which the elemental ions are derived are essentially lost in the ionization process. As a result, ICP's are particularly unsuitable for use in analyzing organic component analytes.
It should be appreciated that high energy electrical discharge is associated with improved sample ionization, hence, increased API sample analysis system sensitivity. In addition, the efficiency of production of sample molecular fragments for use in structural analysis is improved as the power intensity of an electric discharge applied to a sample is increased. Use of very high electrical energy discharge in ICP's, however, is associated with complete fragmentation of a sample and relatively complete loss of sample molecules. A technique which would provide the benefits associated with application of high electrical energy discharge, without the drawbacks associated therewith, would therefore be of great utility. Such a technique is introduced in a patent to Rice et al., U.S. Pat. No. 4,586,368. The Rice et al. system provides an electrodless API discharge system which generates afterglow metastable intermediary species, the energy in which is used at a remote location to controllably fragment and ionize sample. The Rice et al. system, however, does not provide for interfacing to a mass spectrometer, but instead teaches that sample analysis should be by optical means.
The above shows that benefits are available when samples are ionized and fragmented by use of intermediary metastable species produced by API techniques. However, no reference cited teaches an API system and method of use which at once:
allows nebulization and desolvation of sample solution; PA1 provides large numbers of metastable species and nebulized desolvated sample at a common location remote from the location at which metastable species are produced; PA1 increases MS sample analysis system sensitivity by efficiently utilizing energy in metastable species to ionize sample molecules and fragments etc., in a modularly separate ionization system; and PA1 can controllably produce sample molecules or sample fragments etc. in proportions desired by a user. PA1 provide reduced background noise in MS sample analysis system results by limiting nonsample analyte ion production via removal of solvent from nebulized liquid sample prior to ionization thereof and entry into a MS ion detector, PA1 controllably provide ionized sample molecules, and/or sample fragments etc. in proportions as desired by a user thereof; PA1 provide increased MS sample analysis system sensitivity by generating and efficiently utilizing the energy in large numbers of intermediary metastable species in sample fragmentation and ionization processes. PA1 1. M*+A----------M+(A+)+e-; (Penning Ionization) PA1 2. M*+A----------M+(B+)+C+D+e-; PA1 1. (M+)+A--------M+(A+); PA1 2. M*+2HI--------M+(H.sub.2 I+)+(I-); PA1 3. (H.sub.2 I+)+A--------(AH+)+HI;
A need for additional, modular, atmospheric pressure ionization means for use in mass spectrometer sample analysis systems is thus identified which can: